(1) Field of the Invention
This invention relates to an optical communication system and more particularly, to an optical communication system for wavelength division multiplexing (WDM) optical communication.
(2) Description of the Related Art
In recent years Asynchronous Transfer Mode (ATM) networks have been rapidly developed with increasing needs for communication services including the Internet. The ATM is suitable for multimedia communication, with a guarantee of a defined level of Quality of Service (QoS), as well as providing high-speed and wide-bandwidth connections.
Further, WDM optical transmission techniques are widely adopted, which multiplex many signals using different optical wavelengths for simultaneous transmission over a single optical fiber.
The ATM network providing an ATM communication service with such WDM optical transmission techniques is desired to provide more sophisticated services and a higher quality of service for multimedia applications.
The WDM systems achieve long-distance and large-capacity transmission with many optical repeater amplifiers provided on a transmission line. But on the other hand, as an inevitable problem in optical transmission, non-flat wavelength response of optical repeater amplifiers and nonlinear properties of transmission media degrade the signal to noise (S/N) ratio of optical signals being transmitted, depending on their wavelengths. To obtain uniform S/N ratios, optical pre-emphasis is conventionally performed.
The optical pre-emphasis first measures the Bit Error Rate (BER) of an optical signal transmitted via many optical repeater amplifiers. If the result is unsatisfactory, the transmission station performs WDM transmission with an increased power level of the optical signal so that the BER satisfies a prescribed level, typically 10E-15(=10−15). However, in this technique, all WDM wavelengths are subject to optical power control, regardless of what ATM connection services they are used for. This increases the cost of the network.
The currently dominant ATM connection services include Unspecified Bit Rate (UBR) service and Continuous Bit Rate (CBR) service, which are different in levels of QoS. The UBR is a best effort type of service in which cells are sent if there is any free bandwidth. The CBR, on the other hand, is a service in which a bandwidth can be kept like a leased line. UBR service is intended for carrying LAN traffic. If the data transfer fails, a relevant user-side protocol (e.g., TCP) is supposed to retry it. Therefore, the UBR service does not necessarily have to keep a BER of as low as 10E-15, but is allowed to operate with a relaxed quality requirement, such as a BER of 10E-11 (=10−11).
However, as described above, the conventional technique applies optical pre-emphasis to ensure a high transmission quality for all wavelengths, regardless of what connection services (that is, UBR service and CBR service) they are used for. This is an excessive function, and increases power consumption and network costs. We need to find another solution to realize a more economical ATM network.